The present invention relates to combined primary-secondary-tertiary wastewater system and, more specifically, relates to the use of alternate aerobic-anaerobic stage secondary treatment.
In one secondary wastewater treatment system, activated sludge is recycled and microorganisms in the stage consume the wastewater nutrients. In some advanced systems, the effluent is further treated by a biological nitrification-denitrification process for the removal of nitrogen. During nitrification, the ammonia content of the influent is converted to nitrate, utilizing the organic carbon content of the wastewater. During denitrification, the nitrate is converted to gaseous nitrogen, also requiring the use of organic carbon. In many instances, the effluent from nitrification does not contain sufficient organic carbon for effective denitrification. Accordingly it is frequently necessary to add supplemental organic carbon to the denitrification zone, commonly in the form of methanol.
One improvement of the above system is by the use of activated sludge with alternate aerobic and anaerobic stages in which incremental reduction in the organic carbon and nitrogen content of the wastewater is accomplished in each stage. It is possible in this system to maintain the organic carbon in the latter stages at a sufficient level for denitrification without adding an additional source of carbon such as methanol. A system of this type is disclosed in an article by Barnard, J. L. entitled "Biological Nutrient Removal Without The Addition of Chemicals", Water Research (Pergamon Press), Vol. 9, pages 485-490 (1975). Another system similar to the above Barnard system is disclosed in a paper by Drews, J. L. C., et al in Water Research Pergamon Press, 1973, Vol. 7, pp. 1183-1194. This paper discloses that nitrogen may be eliminated from sewage effluent in an orbital system by rapid alternation of aerobic and anaerobic conditions.
One problem with any activated sludge system is the operation of handling large amounts of sludge. Another problem with such systems is that activated sludge is known to be highly sensitive to variations in pH level, temperature, nutrient concentration changes, and toxicity shifts and other biological growth-influencing shifts.
In another type of wastewater treatment system, microorganisms are supported on fixed growth media for digestion of the wastewater nutrients (including carbon). For example, in B. F. Goodrich Information Bulletin VC 2.1-276, there is a disclosure of both aerobic digestion and nitrification in a reactor media in which microorganisms grow in a fixed film on sheet-like media. There is a further disclosure that the same type of media could be used in an anaerobic fixed film reactor for denitrification of previously nitrified wastewaters. However, there is no suggestion of the use in combination of these two different types of reactors.